1. No me gustan los pantalones negros; prefiero los blancos.
In Spanish, the word los pantalones (pants) is masculine (lo instead of la, and negro instead of negra) and always plural (los instead of lo and negros instead of negras). The same thing goes for blancos - it has to be los blancos.
2. Tengo unas botas oscuras. Quiero unas claras.
In Spanish, the word unas botas (boots) is feminine (una for women instead of uno for men, and clara for feminine instead of claro for masculine), and since there are two boots, you have to use the plural form (unas instead of una, and claras instead of clara).
3. Esta camisa es apretada. Prefiero aquella floja.
These sentences mean - This shirt is tight. I prefer a loose one.
In Spanish, the word camisa (=shirt) is feminine (which means you have to use aquella instead of aquello, which is masculine). Since there is no option aquellas flojas, which would also be correct, you have to choose aquella floja, which is singular and correct.
